Information signals, such as media signals and especially compressed video and audio streams and packetized audio and video streams propagate over various communication channels, such as terrestrial, wireless, satellite and cable communication channels and the like. Video streams usually include a relatively large amount of information and therefore are usually transmitted over communication channels in a compressed form.
The Motion Pictures Expert Group (MPEG) specifications are standardized methods for compressing and transmitting media signals such as video and audio.
As media signals, such as media signal streams, sequences of media signals and the like are transmitted over communication channels there is a need to match the bit rate of the media streams to the available bandwidth of the communication channels. This match usually requires to reduce the bit rate of the media streams. A simple conversion of bit rate that does not take into account the content of the media streams can cause severe quality losses. There are some prior art bit rate conversion methods that take into account the content of the media streams and accordingly offer a lower quality loss than the simple conversion of bit rate. Some of the prior art methods are illustrated in U.S. Pat. No. 6,181,711 of Zhang et al.
A disadvantage of prior art methods for bit rate conversion is that the amount of bit rate conversion achieved by their implementation is unknown. Accordingly, in order to match a bit rate of at least one media stream to the available bandwidth of a channel there is a need to either perform some bit rate conversion iterations or to start with relatively drastic bit rate conversion schemes that can cause a larger than necessary bit rate conversion. The bit rate conversion scheme is further complicated when dealing with multiple media streams, as the bit conversion of one media stream affects the available bandwidth for other media streams.
Very often a media stream is bit rate converted various times during a transmission over at least one communication channel. A further disadvantage of prior art systems and methods of bit rate conversions is that each of these bit rate conversions causes an additional delay. Furthermore, each bit rate conversion is performed regardless to previous bit rate conversions.
The problems resulting from bit rate conversion is more acute in systems such as the system illustrated in FIG. 1, in which many systems implement bit rate conversion schemes on the same media streams.
FIG. 1 illustrates a prior art system 9 for generating, transmitting and receiving digitized programs. A plurality of digitized programs, originating from n media sources 10-j, j=1, 2, . . . n are provided to central distribution center 12 that multiplexes the digitized programs to generate at least one primary combined signal. A digitized program includes a plurality of media signals. Usually, central distribution center 12 also performs channel specific encoding and modulation for allowing the transmission of the primary combined signal. The at least one primary combined signal is transmitted to at least one distribution satellite, such as distribution satellite 14. Distribution satellite 14 re-transmits the at least one primary combined signal to m headends 20-k, k=1, 2, . . . m. Usually, distribution satellite 14 has a transponder for converting the frequency of the at least one primary combined signal. Accordingly, a primary combined signal is also referred to as a transponded signal.
Additional control, status and billing information are usually transferred over auxiliary cable networks, such as auxiliary cable network 16.
Headend 20-k is configured to receive at least a single primary combined signal, and to provide at least a selected portion of the received primary combined signal to a plurality of subscribers, such as set top boxes 28-l, l=1, 2, . . . t via cable network 30. The selected portion is also referred to as a secondary combined signal and includes at least one selected digitized programs. The secondary combined signal can also be retransmitted to a local distribution satellite (not shown) via a local uplink transmitter, such as transmitter 26.
Each headend is configured to select at least some digitized programs in view of predefined selection information, to analyze the selected digitized programs and to convert the bit rate of some portions of the selected digitized programs in view of the available bandwidth of the downstream channel to which the secondary combined signal is to be provided. The secondary combined signal includes the selected digitized programs, some of which were bit rate converted.
Methods for selecting digitized programs and generating a combined secondary packetized stream are known in the art. The selection is also known as grooming.
Referring to FIG. 1, each headend includes an analyzer (A) 20-k-2, a controller (C) 20-k-4 and a processor (P) 20-k-6, for performing the selection, bit rate conversion and combination process. Analyzer 20-k-2 analyses the selected digitized programs to determine the required bit rate for a transmission of each digitized program, and generates information that reflects the amount of either actual or predicted bit rate conversion that can be achieved by applying bit rate conversion techniques. Said analysis can involve a full or partial decoding process of the digitized program. The results of the analysis are provided to controller 20-k-4 that also receives bandwidth information being indicative of the available bandwidth of the downstream channel. In view of the additional information controller 20-k-4 determines whether to apply bit rate conversion techniques, which technique to apply and to which of the selected digitized programs to apply the bit rate conversion techniques. Controller 20-k-4 provides processor 20-k-6 instructions that reflect the determination and processor 20-k-6 generates the secondary combined signal. The secondary combined signal has a bit rate that is either substantially equal or less than the available bandwidth of the downstream channel through which the secondary combined signal is to be transmitted.
Methods for combining various signals to use an available bandwidth are known in the art. A common method for combining the selected digitized signals is known as statistical multiplexing. U.S. Pat. Nos. 6,192,083, 6,094,457 and 6,038,256 of Linzer et al, describe apparatuses and methods for statistical multiplexed encoding using pre-encoding a-priori and a-postriori statistics. In said patents the analyzer is referred to as either a statistics gatherer, or as a combination of a statistical gatherer and a plurality of encoders.
A single digitized program is received by many headends. A single distribution satellite usually transmits the primary combined signal to thousands of headends. Accordingly, many analyzers analyze the same digitized program. The analysis process is relatively complicated and costly, and directly affects the cost of the headend. The inclusion of an analyzer within each headend is both costly and not effective.
Statistical multiplexing of more than one digitized program is further complicated as the amount of bit rate conversion resulting from an appliance of bit rate conversion schemes on the digitized programs can not be accurately predicted. Accordingly, more than a single bit rate conversion scheme must be applied on the digitized programs before the bit rate conversion results in a match between the bit rate of the secondary combined signal and the available bandwidth of the downstream channel.
A further disadvantage of the prior art methods and apparatuses is that the complexity of each analyzer has to be restrained, in order to reduce the cost of the headend.
There is a need to provide cost effective system and method for performing bit rate adaptation and multiplexing.